Rapid solidification of martensitic stainless steel atomized droplets

The microstructure of martensitic stainless steel powders produced by inert gas atomization was investigated. Depending upon the powder particle size, the microstructure was found to exhibit a cellular, dendritic, or martensitic morphology. Relationships between the microstructure scale and the particle diameter were identified. It was found that at a critical particle diameter of 25 to 30 μm, the structure changed from cellular/dendritic (96.5 vol pct bcc and 3.5 vol pct fcc) to martensite. The solidification path of the powder particles below and above 25 to 30 μm in size was considered. High-temperature X-ray diffraction (HTXRD) measurements revealed that there is a delay in the appearance of the fcc phase for the small particle size. The delay in the appearance of the fcc phase is a result of different nucleation sites for the fcc phase between the large and the small particle size.     

Deformation of martensite in a polycrystalline Cu-Zn-AI alloy

The deformation morphologies and mechanisms in polycrystalline martensitic Cu-Zn-AI samples have been examined. A variety of deformation morphologies are exhibited simultaneously in moderately cold-worked polycrystalline samples. The mechanisms by which these are developed have been ascertained, and include variant-variant coalescence, stress-induced martensite-to-martensite transformation, injection of foreign variants to plate groups, internal twinning, and slip. The crystallographic features of the various deformation morphologies have been completely characterized using transmission electron microscopy. Transformation peaks monitored by differential scanning calorimetry showed tails on the completion side, and diminished peak size, for cold worked specimens. These effects are correlated with observed microstructural features.     

Rapid solidification of a droplet-processed stainless steel

Individual powder particles of a droplet-processed and rapidly solidified 303 stainless steel are characterized in terms of microstructure and composition variations within the solidification structure using scanning transmission electron microscopy (STEM). Fcc is found to be the crystallization phase in powder particles larger than about 70 micron diameter, and bcc is the crystallization phase in the smaller powder particles. An important difference in partitioning behavior between these two crystal structures of this alloy is found in that solute elements are more completely trapped in the bcc structures. Massive solidification of bcc structures is found to produce supersaturated solid solutions which are retained to ambient temperatures in the smallest powder particles. Calculated liquid-to-crystal nucleation temperatures for fcc and bcc show a tendency for bcc nucleation at the large liquid supercoolings which are likely to occur in smaller droplets. The importance of small droplet sizes in rapid solidification processes is stressed. Formerly with Massachusetts Institute of Technology, Cambridge, MA.     

Rapid solidification processing of a Mg-Li-Si-Ag alloy

A Mg-13Li-4Si-lAg (wt pct) alloy with improved ductility and thermal stability was developedvia the rapid solidification (RS) processing technique. Silicon was added to the alloy as the third alloying element in order to form a thermally stable intermetallic dispersoid phase required for improved mechanical properties at ambient and elevated temperatures. The microstructure of the as-spun and heat-treated alloy was characterized using differential scanning calorimetry (DSC), X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Microhardness measurements were conducted on as-spun and heat-treated alloy in order to obtain qualitative prop-erty data and to investigate the extent of the degradation of properties at elevated temperatures. It was found that the melt-spun Mg-Li alloy possessed a microstructure consisting of a fine dispersion of Mg₂Si phase in a fine-grained body-centered cubic (bcc) Mg-Li solid solution, resulting in the desired improvements in thermal stability and mechanical properties. Formerly Graduate Student, Department of Materials Science and Engineering, University of California     

铝合金中镁的快速测定

所介绍的铝合金中镁的快速测定,采用DDTC、盐酸羟胺、三乙醇胺联合掩蔽剂掩蔽大量的铝及少量的铜铁锰离子,在PH10以铬黑T为指示剂,用EDTA滴至溶液由紫红色变为纯兰色为终点。方法简单,快速,准确度高,适合于铝合金中镁含量的测定,是一种较好的铝合金中测定镁的方法。     

The stabilization of martensite in Cu-Zn-AI shape memory alloys

The martensitic transformation temperature in shape memory alloys can be affected differently by aging above and below the transformation temperature. Under such circumstances the normally reversible transformation can be prevented and the martensite structure “stabilized”. This effect has been studied using electron microscopy, differential scanning calorimetry, and mechanical testing. Evidence is given of an apparently martensitic high temperature transformation, and a careful comparison is made of the stabilized and unstabilized states of the alloy. Three possible models for stabilization are considered in the light of the results obtained, and it is concluded that no single mechanism can be responsible for all the phenomena observed. Formerly with the Department of Metallurgy and Materials Science, University of Cambridge, England     

Rapid solidification characteristics in melt spinning a Ni-base superalloy

The solidification kinetics involved in the process of melt spinning a Ni-base superalloy have been characterized. Through a correlation of ribbon thickness to melt puddle residence time, it was found that the solidification front velocity,V, is typically about 100 mm per second at the ribbon surface not in contact with the spinning wheel. The rate of solidification varies within the ribbon, increasing with decreasing distance,S, from the wheel-contact surface asV = 3.65S ^-1. Ribbon microstructure and texture characteristics are discussed in light of this kinetics result. The thickness-vs-time correlation was further analyzed to yield information about thermal history during ribbon formation. These thermal results are generally consistent with those deduced from dendrite arm spacing measurements. Formerly with General Electric Research and Development Center.     

Nucleation and growth of bainite crystals in Cu-Zn-AI alloys

The bainitic transformation in Cu-Zn-Al alloys is known to have both martensitic and diffusion-controlled natures. To study the relation between these two reactions involved in the bainitic transformation, the present authors have performed a series of investigations mainly by optical and electron microscopy. A mechanism whereby directional diffusion of solute atoms stimulates the nucleation and growth of bainite is proposed based upon these investigations. The mechanism is supported by an additional experiment in which a local solute concentration in the matrix crystal is directly measured around the bainite plates using a special analytical electron microscope. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Rapid melting and solidification of a surface layer

Metallurgical and Materials Transactions B - A one-dimensional computer heat flow model is used to investigate the effect of high intensity heat fluxes,e.g. those achieved via continuous CO2 laser...     

Rapid melting and solidification of a surface layer

A one-dimensional computer heat flow model is used to investigate the effect of high intensity heat fluxes,e.g. those achieved via continuous CO2 laser radiation, on the important surface layer melting and subsequent solidification variables of three substrate materials: aluminum, iron, and nickel. Temperature profilesvs time, melting, and solidification interface velocities, heating, and cooling rates in the surface layers of the three metals are calculated. Results are presented in a general form to permit determination of these variables for large ranges of absorbed heat fluxes and times. General trends established show that temperature gradients in the liquid and solid phases and interface velocities are directly proportional to the absorbed heat flux, whereas melt depth is inversely proportional to the absorbed heat flux. Average cooling rates comparable to splat cooling can be achieved by increasing the heat flux and reducing the dwell time of the incident radiation. An order of magnitude increase in the absorbed heat flux results in a corresponding two orders of magnitude increase in average cooling rates in the liquid during solidification of crystalline and noncrystalline structures. Formerly Research Associate, Formerly Research Associate, Formerly Research Associate,     

定向凝固过程中的流动效应

在类合金(NH₄Cl-H₂O溶液)定向凝固晶体生长实验装置上,利用ϕ30μm煤粉作示踪粒子,再现糊状区内微通道流以及通道出口处的流体流动,并测算了各处流体的瞬时速率.分析认为:凝固初期,糊状区内固相体积分数较大,内部流体流动受阻;随着固相体积分数减少,糊状区孔隙率增大,流体充分发展;当平均固相体积分数降至0.42,接近最小值0.38时,当量雷诺数达到临界值(247),糊状区内形成微通道;随着通道宽度逐渐扩大,液相区内热流体进入微通道.微通道内稀冷液体向上流,浓热液体向下流,促使通道内溶液再结晶.     

Rapid solidification of Al-Cu eutectic alloy by laser remelting

Rapid surface resolidification using a high powered CO₂-laser has been performed on eutectic Al-32.7 wt% Cu at speeds between 0.2 and 8 m/s. By means of longitudinal cuts through the centre of the laser trace, the local growth rate has been measured by observation of the orientation of the microstructure using transmission electron microscopy. The various microstructures as a function of growth rate, are:

• 1.(a) regular lamellar eutectic α-Al/θ-Al₂Cu structure for growth rates below 20 cm/s with interlamellar spacing as fine as 17 nm;
• 2.(b) a new wavy eutectic α-Al/θ'-Al₂Cu morphology for growth rates of between 20 and 50 cm/s;
• 3.(c) a banded structure formed by alternating supersaturated α-Al solid solution and the wavy eutectic for growth rates greater than 50 cm/s.

A recent analytical model for eutectic growth under rapid solidification condition is compared to the experimental results. Contrary to the classical λ²V_s = const. relationship, which predicts a continuous decrease in spacing as the growth rate increases, this new theoretical model clearly predicts a limit for the coupled eutectic growth which finds its analogy in single phase solidification in the limit of absolute stability.     

Nanocomposites of Aluminum Alloys by Rapid Solidification Processing

Aluminium alloys reinforced with transition metal aluminide (Al₃Ti, Al₃Fe, Al₃Ni, etc.) particles possess high specific strength both at ambient and elevated temperature. The improved strength of these alloys are the results of slower coarsening rate of the intermetallic particles due to low diffusivity of the transition metals in aluminium. However, the strength can be enhanced further by refining the microstructure of the alloys to nanometer range. The authors have successfully attempted two important non-equilibrium processing techniques i.e. rapid solidification processing (RSP) and mechanical alloying for the refinement of the microstructure in various aluminium alloys. In this report, authors present a short review of their work on RSP of Al?CTi and Al?CFe alloys to produce nanocomposites.     

水溶液氯化法高效溶解铂铑合金

我国工业上主要采用湿法工艺中的王水溶解法回收铂族金属,但由于其具有溶解时间较长、溶解温度较高的缺点,需要对该工艺进一步优化。以铂铑合金粉末(约含铑10%,铂90%)为原料,通过在自制反应釜中向传统王水溶解体系通入氯气来提高王水氧化铂铑合金的能力,并通过加入铁基离子液体富集反应产物PtCl_6~(2-)和RhCl_6~(3-),从而实现铂铑合金的低温快速溶解。考察了盐酸/硝酸体积比、液固比、液氯用量、反应温度、离子液体用量对铂铑合金溶解速率的影响。结果表明,在最佳反应条件下,铂铑合金全溶时间缩短至1.52h;通过对铂铑合金溶解液进行分离提纯后,得到纯度大于99.95%的铂、铑单质,回收率分别为99.2%和98.6%。对传统王水溶解工艺改进后,溶解效率提升了约78.29%,本方法在工业上应用具有很大潜力。     

Thermal effects in rapid directional solidification: linear theory

We study the morphological instability of the planar solid/liquid interface for a unidirectionally-solidified dilute binary mixture. We use a model developed by Boettinger et al., Aziz, and Jackson et al., which allows for nonequilibrium effects on the interface through velocity-dependent segregation and attachment kinetics. Two types of instabilities are found in the linear stability analysis: (i) a cellular instability, and (ii) an oscillatory instability driven by disequilibrium effects. Merchant and Davis characterized these instabilities subject to the frozen-temperature approximation (FTA). The present work relaxes the FTA by including the effects of latent heat and the full temperature distribution. Thermal effects slightly postpone the onset of the cellular instability but dramatically postpone the onset of the oscillatory instability; however, the absolute-stability conditions, at which at high speed the cellular and oscillatory instabilities are suppressed, remain unchanged from the FTA. The critical wavenumber for the oscillatory instability can be zero or nonzero depending on the material parameters. The experimental observations of banding are correlated with the predictions of our theory.     

Natural convection transients and their effects in unidirectional solidification

A formulation is given and computed solutions are presented for transient solidification accompanied by natural convection in a vertical slot. It was found that appreciable fluid velocities may be produced by natural convection, the values of which could be comparable to the terminal rising velocities of typical nonmetallic inclusions. The simplifying assumptions made limit the validity of the solutions to systems where GrPr < 500,i.e., to narrow slots or to low values of the superheat; nonetheless, the results should be indicative of the effects of convection at much higher values of GrPr. Formerly Post-Doctoral Research Associate, State University of New York at Buffalo